In dental care, general imaging in which film is positioned in back of teeth to perform X-ray imaging, panoramic imaging which involves simultaneously rotating an X-ray tube and film, cephalometric imaging in which an X-ray tube is greatly detached from the film during imaging, and the like are available. However, because all of these procedures involve simple X-ray imaging, only a plane image is obtainable.
Recently, a new type of dental surgery has been performed in which an implant is embedded into bone to fix artificial teeth to the jaw. When such surgery is performed, it is important to three-dimensionally observe the form of the teethridge and nearby bones. However, according to conventional X-ray imaging, it is impossible to three-dimensionally observe the form of teethridge and nearby bones, and so the form has to be determined on the basis of a plane image to perform surgery. Accordingly, the surgery requires more than normal skill, and there has been a problem in that a medical accident is likely to happen.
Such a problem can be solved by three-dimensionally observing the form of the teethridge and nearby bones using a conventional X-ray CT apparatus. The conventional X-ray CT apparatus does not image only one portion of a section of a body, such as a teethridge and nearby bones, but radiates X-rays to the whole area and obtains a tomogram of a desired portion from the obtained whole-area image. However, because the desired portion which forms the subject here is the form of the teethridge and the nearby bones, the image thereof becomes small compared with the entire image. Therefore, when the desired portion is small, the extracted image is small, and it is difficult to be recognize details since the image thereof is extracted from the entire image.
In this case, a tomogram of the desired portion can be easily imaged by constructing an X-ray CT apparatus which operates to radiate X-rays only to the desired portion and to create a projection image thereof. One example of an apparatus which radiates X-rays only to a desired portion to create a CT image thereof is described in Japanese Unexamined Patent Publication 2000-139902.
According to Japanese Unexamined Patent Publication 2000-139902, because X-rays are locally radiated to a local portion by using an X-ray cone beam collimator, prevention of excessive radiation exposure of the object is taken into consideration. However, in operations for selecting whether a tomogram of one tooth being a lesion is created or that of a whole jaw is created, for example, the collimator of an X-ray generator has to be set in every imaging operation, which makes the selecting operation complicated. Therefore, it is difficult to image an arbitrary portion of arbitrary size in an object. This problem is not taken into consideration at all in Japanese Unexamined Patent Publication 2000-139902.
An object of the present invention is to provide an X-ray CT apparatus which can image an arbitrary portion of an arbitrary size in an object.